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The yui is all front end stuff. You can use firebug to see how requests are passed to the backend scripts. yui-ext is a javascript library and will work with whatever backend you have, whether it's php, asp, java....
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is there any trouble using yui-ext and asp.net 1.1?

I have now the the example http://www.jackslocum.com/blog/examples/layout6.htm in aspx I mean I have layout6.aspx and it work OK, but now I set an asp.net button into aspx